Acoustic wave imaging apparatus and method

ABSTRACT

An acoustic imaging apparatus and method that achieves desired delays with coded signals. Linear, curved linear and sector scanning is provided in 1-D arrays and planar, curved planar and sector scanning is provided in 2-D arrays. Composite and non-linear implementations are presented. Dynamic and discrete dynamic focusing is disclosed for the relevant arrays. The 2-D array makes possible 3-D imaging.

FIELD OF THE INVENTION

The present invention relates to acoustic wave imaging systems.

BACKGROUND OF THE INVENTION

Conventional acoustic wave imaging systems use a one dimensional (1-D)array of electro-acoustic transducers, for example, a 1×100 array, andhave been configured to achieve linear, curved linear and sectorscanning. Coherence in the transmission and receipt of acoustic signalsis achieved by the utilization of delay devices in the signal processingchannels. Present one dimensional systems are disadvantageous due to (1)the manner in which they are constructed and (2) inherent limitations intheir scanning capabilities. With respect to the manner in which theyare constructed, one disadvantage is that the use of delay elements, andrelated electronics adds considerably to the cost of one dimensionalsystems. With respect to inherent limitations, one dimensional scanningsystems are disadvantageous in that they only provide two dimensionalimages.

To increase diagnostic capabilities it is desirous to have an acousticimaging system that scans in two dimensions and thus produces a 3-Dimage. A problem with applying current 1-D technology to 2-D arrayimaging is that a vast number of electrical connections and processingelectronics are required to serve an array of practical size. Forexample, a 100×100 array would have 10,000 individual transducers.Standard technology would require 10,000 electrical connections andprocessing channels. At an approximate cost of $100 per channel, such asystem would require an outlay of $1M merely for channel electronics. Inaddition, if per channel power consumption is approximately 0.1 watt,then the system power requirement becomes at least 1 KW.

As a result of the disadvantageous aspects of providing large numbers ofprocessing channels, current research efforts are directed towardsachieving high performance with fewer array elements. Two prior artapproaches are (1) the use of a two dimensional array with a reducednumber of columns, termed a 1.5-D array, and (2) a thinned 2-D array.

A typical embodiment of a 1.5-D array is a 100 row×3-5 column array. Dueto reduced aperture, 1.5-D arrays may not offer the increase inelevation resolution that will justify their added expense andcomplexity. Furthermore, research has shown that the use of aberrationcorrection with 1.5-D arrays may not improve the image quality over thatobtained using correction with a 1-D array.

In a thinned array, the number of array elements and associatedelectronics is reduced to several hundred by judiciously using only aselected number of transducers throughout the array aperture. Thisapproach, however, has lead to significantly higher sidelobes in thebeam profile of the system, compared to sidelobes in a beam profile fora full 2-D array. Thus, these systems are not suitable for such commonuses as medical diagnostic imaging and the like which requires low andextremely low level sidelobes.

It should also be noted that the prior art does include 2-D annulararrays. These arrays are formed of concentric annual rings. They produceonly a single ray, and while they perform dynamic focusing, they do notprovide sector scanning. Scanning is achieved by physically moving thearrays.

In view of the foregoing, it should be apparent that a need exists inthe art for both (1) a more economically constructed one dimensionalscanning system, and (2) an acoustic wave imaging system that providesthe high degree of resolution achievable with a full 2-D array, whilereducing the number of processing channels and other circuitryassociated therewith, amongst other needs.

SUMMARY OF THE INVENTION

Accordingly, it is the object of the present invention to provide anacoustic imaging system having a 1-D array that uses coded signals toachieve requisite delays.

It is another object of the present invention to provide an acousticimaging system having a 2-D array that has a number of processingchannels that is less than the number of transducers.

It is another object of the present invention to provide a multiplicityof embodiments and implementations for these 1-D and 2-D acousticarrays.

It is also an object of the present invention to transmit and/or receiveacoustic energy with these 1-D or 2 -D acoustic arrays.

These and related objective objectives of the present invention areachieved by use of the acoustic wave imaging system and method describedherein.

Amongst other aspects, the present invention discloses a manner of usingcoded signals to achieve desired delays in 1-D, 2-D and annular arrayimaging systems. For 1-D imaging array systems, a manner of making andpracticing linear, curved linear and sector scanning arrays that are lowcost and potentially portable is presented. This teaching also appliesto annular arrays. For 2-D array imaging systems, a manner of making andpracticing these systems for rapid 3-D volume images and/or real time,arbitrary scan plane, 2-D sector, planar, or curved planar images ispresented. In addition, the 2-D array taught herein allows for theimplementation of multi-dimensional aberration correction which mayallow ultrasonic images to approach the image acuity of MRI and X-rayCAT imaging systems, and to do so at a fraction of the cost of theseimaging modalities.

Both dynamic focusing and discrete dynamic focusing is taught for thesystems herein. Composite and non-linear implementations, as definedherein, are also disclosed.

In one embodiment, the present invention comprises a plurality ofelectro-acoustic transducer transducers, each capable of generating anelectrical signal indicative of an incident acoustic wave; means incommunication with each transducers transducer for generating a codedsignal for transmission by each of said transducers; and means incommunication with each of said transducers for modifying a coded signalreceived by the transducers to achieve a desired delay.

The present invention also comprises a plurality of electro-acoustictransducers, each capable of generating an electrical signal indicativeof an incident acoustic wave and arranged in an array; control means incommunication with each of said transducers and having a plurality ofcontrol channels for controlling said transducers; and means incommunication with each of said transducers for processing image datatherefrom; wherein said plurality of control channels is fewer in numberthan said plurality of transducers.

The invention also includes an transducer element comprised ofnon-linear electro-acoustic, non-linear dielectric material.

And in yet another of many embodiments, the present invention includesan array of electro-acoustic transducers having a plurality of rows andcolumns; a plurality of row control lines, each of which is coupled tothe transducers in one of said plurality of rows; a plurality of columncontrol lines, each of which is coupled to the transducers in one ofsaid plurality of columns; and control means coupled to each of saidplurality of row and column control lines for generating a controlsignal for each transducers transducer that is a combination of controlsignals on the row and column control lines for that transducer.

Methods for realizing a desired delay and for achieving planar, curvedplanar and sector scanning are also disclosed.

The attainment of the foregoing and related advantages and features ofthe invention should be more readily apparent to those skilled in theart, after review of the following more detailed description of theinvention taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an acoustic wave imaging system inaccordance with the present invention.

FIG. 2 is a schematic/block diagram of an acoustic transducer array andcontrol circuits therefor in accordance with the present invention.

FIG. 3 is a block diagram of interface circuitry for an acoustic waveimaging system in accordance with the present invention.

FIG. 4 is a diagram of a chirp signal.

FIG. 5 is a diagram of planar scanning in accordance with the presentinvention.

FIGS. 6a-6b are diagrams of curved planar arrays in accordance with thepresent invention.

FIG. 7 is a schematic diagram of two cells of FIG. 2 with phaseadjustment in accordance with the present invention.

FIG. 8 is a diagram illustrating aspects of discrete dynamic focusing inaccordance with the present invention.

FIG. 9 is a range point versus frequency band diagram in accordance withthe present invention.

FIG. 10 is a block diagram of a phase accumulator in accordance with thepresent invention.

FIG. 11 is a block diagram of a modified receive channel in accordancewith the present invention.

FIG. 12 is a schematic/block diagram of an embodiment of a 1-D array inaccordance with the present invention.

DETAILED DESCRIPTION

The present invention includes both 1-D and 2-D array acoustic imagingsystems. Both of these systems may be realized in a multiplicity ofembodiments and may be implemented in different material.

For the 2-D array scanning system at least 3 practical embodiments arecontemplated. They are the planar array, the curved planar array, andthe sector scanning embodiments. At least two different implementationsare also contemplated. They are the composite electronic/acousticimplementation (hereinafter referred to as the “composite”implementation) and the non-linear electro-acoustic, non-lineardielectric implementation (hereafter referred to as the “non-linear”implementation. One difference between the composite and non-linearimplementations is that a mixing function (taught below) is provided inan electronic circuit fabricated in semiconductor material in thecomposite implementation, while in the non-linear implementation, thatsame mixing function is achieved as a characteristic of the selectednon-linear material. As will be described in more detail below, theplanar and curved planar array can be achieved in both composite ornon-linear implementations, while sector scanning is achievable in thecomposite implementation.

The 1-D system is in large part a subset of the 2-D system and hence,linear scanning, curved linear scanning, and sector scanning may berealized. The teachings herein for the two different implementations of2-D arrays applies likewise to 1-D arrays.

The 2-D and 1-D scanning systems are now presented in more detail. Ageneral overview of a 2-D system is presented first in which a 1-D arraymay be substituted with apparent variation, followed by a description ofsystem operation. Discussion of more specific embodiments is thenprovided, including both (i) achieving ranging in angular scanning and(ii) configuring a 1-D system, amongst other aspects.

Referring to FIG. 1, a perspective view of an acoustic wave imagingsystem 10 in accordance with the present invention is shown. The system10 includes interface circuit 20 which is connected via line 85 tooperator interface componentry represented by reference numeral 80 andvia line 75 to a display mechanism 70. Both the operator interfacecomponentry 80 and the display mechanism 70 are known in the art and arediscussed in more detail below with reference to FIG. 3. The interfacecircuit 20 is also connected, via line 22, to a row control circuit 30and, via line 28, to a column control circuit 40. The row and columncontrol circuits 30,40 control the phase and frequency of signalspropagated to a plurality a of M rows and N columns in an array 100 ofacoustic transducer elements. Each transducer element comprises aacoustic transducer (cells 110, 120, 140,170,180,190 are indicated inFIG. 1) and its includes a corresponding transducer (shown in FIG. 2).The row control signals are propagated over M row control lines orprocessing channels, represented generally by arrow 35, and the columncontrol signals are propagated over N column control lines or processingchannels, represented generally by arrow 45.

Acoustic waves incident or transducers in array 100 cause the generationof a corresponding electrical signal that is combined with the row,X-axis, and column, Y-axis, control signals in each transducer cell andthen combined with the output of all other cells before propagation overline 65 to interface circuit 20 in a manner discussed below. Ininterface circuit 20, the signal is processed for imaging and output vialine 75 to display mechanism 70 for display. The interface circuit 20 isdiscussed in more detail below with reference to FIG. 3.

Referring to FIG. 2, a schematic/block diagram of array 100 and row andcolumn control circuits 30 and 40, respectively, is shown (the array ispresented as a schematic and the control circuits as block diagrams).The array is comprised of M rows and N columns and a transducer ispreferably located proximate the intersection of each row and columnsignal line. Conventional 1-D arrays often contain 64 or 128 linearlyarranged transducers. Accordingly, the array 100 is anticipated to havea size ranging from 64×64 to 128×128 transducers, and thus anapproximate size of 100×100 is made reference to herein. In planar andcurved planar scanning, discussed below, the array size may be muchlarger, for example 200×200 to 500×500 or a rectangular combinationthereof, of which only a sub-unit is active at any given time. Forexample, the planar array may be 400×400 transducers, with a subapertureof 100×100 transducers that is active at a given time. If the embodimentof FIG. 2 has 100 row lines and 100 column lines, then 10,000transducers are supported.

In the composite implementation the transducers are standardelectro-acoustic transducers. They are processed to a particular sizethat affords an appropriate center frequency and bandwidth. In apreferred embodiment, those parameters are respectively 5 MHz and 4 MHz.

FIG. 2 illustrates 9 transducer cells 110(not labelled in FIG. 2 due tocrowding in the figure, but labelled in FIG. 1) ,120,130,140,150,160,170,180,190 and their corresponding acoustictransducers 115,125,135,145,155,165,176 175,185,195. In the compositeimplementation, each transducer is mounted to its corresponding cell inthe same manner that transducers are connected to semiconductorsubstrates in IR focal plane arrays or the like. The dotted lines areprovided to indicate that the number of cells is variable and may bemodified in either dimension. Cell 150 is surrounded by a dashed lineand will be described as a representative cell.

Cell 150 includes a first mixer 151 for mixing row and column controlsignals in a manner described below. This mixer is a standard highquality electronic mixer and is preferably doubly balanced. The outputof mixer 151 is input to a transmit amplifier (hereinafter referred toas “buffer”) 152 which in turn is connected to a transmit and receive(T/R) switch 153. The T/R switch 153 is controlled by interface circuit20 (connection not shown, but known in art) and is connected to both theelectro-acoustic transducer 155 and an amplifier 157. When an acousticwave is received at transducer 155, a corresponding signal is propagatedthrough T/R switch 153 to amplifier 157. The output of amplifier 157 isconnected to a second mixer 158 which combines the corresponding signalwith the output of first mixer 151. The output of second mixer 158 isconnected to the output of the second mixers 128,188 from each of theother cells 120,180 in the same column via line 102. The combined secondmixer output signals from each column (line 101 provides the combinedsecond mixer signal for mixers 118,148,178 and line 103 provides thecombined second mixer signal from mixers 138,168,198) are connected atpoint 199 and transmitted to interface circuit 20 (FIG. 1) on signalline 65.

The components of cell 150 are provided in the other cells and areidentified therein by both a similar geometric symbol and referencenumerals that use the same number in the units digit. For example, thefirst mixer 151 of cell 150 is identified as 111 in cell 110, 121 incell 120, etc. It should be noted that although a buffer, a T/R switchand an amplifier are provided in each cell to improve signalcharacteristics, their use is not required to achieve the mathematicalsignal processing described herein.

The row control circuit 30 consists of a plurality of individual rowsignal generating circuits 231. A first of these in is connected vialine 251 to the first mixer of cells 110,120, 130. Similarly, a secondand a last row signal generating circuit 231 are connected via lines 252and 253 to cells 140,150,160 and cells 170,180,190, respectively. Thecolumn control circuit 40 consists of a plurality of individual columnsignal generating circuits 241. A first of these in is connected vialine 261 to the first mixer of cells 110,140, 170. A second and a lastcolumn signal generating circuit 241 are connected via lines 262 and 263to cells 120,150,180 and cells 130,160,190, respectively.

The row and column control are connected to the system control circuitry300 (of FIG. 3) and provide frequency and phase modified signals inaccordance with equations below, e.g., chirps on transmit, continuouswaves during receive, and multiple component large bandwidth signals forsector scanning range focusing. In one embodiment, the row and columncontrol circuits 30, 40 include frequency generators and phase shifterswhich are generally known in art and which receive initial values andcontrol signals from interface circuit 20. Amplitude control may also beprovided to improve signal processing and to correct for frequencydependent attenuation in the body.

Referring to FIG. 3, a block diagram of an interface circuit 20 inaccordance with the present invention is shown. The interface circuit 20includes system control circuitry, designated by block 300, thatcommunicates over line 85 with the operator interface 80, over line 351with a detector 350, over line 371 with an image processor 370, line 361with image memory 360, and over lines 22 and 28 to the row and columncontrol circuits (30,40 of FIG. 1). Operator interface 80 is generallyknown in the art and may include a key board and control knobs or thelike for the entry of time, gain and control signals, patient data,transmit power, etc. Furthermore, circuitry and software forimplementing the timing, signal generation, signal processing andrelated functions of the system control circuitry 300 is also generallyknown. It may include a system clock for synchronizing timing and othercontrol signals, a microcontroller or equivalent discrete circuitry andrelated logic. Designing a program and logic to implement the equationsherein would be apparent to one skilled in the art given the teachingsherein.

The output signal from array 100 is propagated over line 65 to a filter310 and preferably to an analog to digital converter (ADC), representedcollectively as 310. This circuit may also include signal conditioningcircuitry. In a preferred embodiment for broadband operation, a matchedfilter 320 (discussed below) is connected to the output of the bandpassfilter/ADC 310 and the matched filter 320 is in turn connected to adetector 350. The detector 350 is provided to convert a signal outputfrom the matched filter 320, which may potentially be 2-3 sinusoidalcycles, into a single unipolar pulse. A quadrature detector is preferredfor Doppler processing.

Outputs from the detector 350 are propagated over line 354 and 355 toimage memory 360 which in turn is connected to the image processor 370.Control signals from system control 300 are generated and propagatedover lines 351,361,371 to detector 350, image memory 360, and imageprocessor 370, respectively, in a generally known manner to result inthe propagation of a display signal on line 75 to display mechanism 70.The display mechanism 70, may be a monitor, a projection screen,stereoscopic glasses, an image recording device, or any other displaydevices that is capable of displaying two or three dimensional images.Circuitry for implementing filter/converter 310, matched filter 320,detector 350, memory 360 and image processor 370 are generally known inthe art. It should be recognized that while an ADC is preferred at 310,the same designated signal processing up until the image memory 360 maybe performed with known analog circuitry.

In operation, data from detector 350 is stored in memory 360 in such amanner that it is read out by image processor 370 and propagated todisplay mechanism 70 in the same manner that data is propagated for aCRT, for example, in a 2-D imaging system, or stereoscopically for 3-Dimaging.

Referring to FIGS. 1-3 collectively, acoustic signal focusing requiresgeneration of a coherent wavefront during transmission mode and receiptof a coherent wavefront during receive mode. Such coherence is achievedby delaying the timing of transmit or receive signals an appropriateamount for a particular transducer based on its location in array 100.The delay is achieved through the use of coded signals. For purposes ofthe present invention, a coded signal is defined as any signal in whicha change in a measurable characteristic thereof, e.g., frequency orphase, results in a change in time delay at a matched output therefor.One suitable coded signal is a linear FM chirp, which is taught hereinin conjunction with a matched filter.

Referring to FIG. 4, a frequency versus time diagram is shown for alinear FM chirp. Chirps as a characterized electrical signal and matchedfilters therefor are generally known. Though an up chirp is shown itshould be recognized that since the attenuation of sound is stronglydependent on frequency, a down chirp may also be used and may be moreappropriate in some instances. Furthermore, it may also be appropriateto transmit high frequencies at a higher voltage level.

For imaging purposes, a chirp must be converted into a pulse beforedetection and display. This is accomplished by way of the matched filter320. A matched filter is a filter whose frequency response is thecomplex conjugate of the frequency spectrum of a signal to be “matched.”By shifting the frequency of the incoming linear FM chirp, the outputpulse of the matched filter will vary in the time that it exits filter320 (and 720 at FIG. 11). This delay, is directly proportional to theshift of the chirp signal away from its original center frequency. Thus,with an appropriately designed matched filter, variable time delays canbe implemented via frequency shifts.

Using chirps (or other suitable coded signals), the frequency of everyrow and column signal is chosen such that the resulting shift infrequency will give rise to the appropriate time delay once the chirp iscompressed into a pulse by a matched filter.

Composite and Non-Linear Implementations

The array 100 has at least two implementations. A first is the compositeimplementation where essentially all the components of each cell arefabricated in semiconductor material and the electro-acoustic transduceris connected thereto. A second implementation involves selecting anappropriate non-linear electro-acoustic, non-linear dielectric material(discussed immediately below) that performs the necessary mixingfunctions of one or both of the first and second mixers as acharacteristic property thereof. With respect to the depiction of array100 in FIG. 2, it should be recognized that the symbols for the firstand second mixers represent, in the first implementation, physicalmixers, and in the second implementation, functions that are beingperformed by the non-linear electro-acoustic, non-linear dielectricmaterial. In addition, in the second implementation, the otherelectronic components, i.e., the buffer, T/R switch and amplifiers arenot provided in the physical array, but amplifiers and the like areprovided off board.

One reason for pursuing the non-linear implementation is that it is morecost effective. The quadratic relationship of the applied voltage tomechanical strain of an electrostrictive transducer can be used to mixthe row and column control signals. Such an array can be constructedwith rows of electrodes connected to one face of all the transducers andcolumns of electrodes connected to the other face (i.e., back). In apreferred embodiment, the non-linear electro-acoustic, non-lineardielectric material is electrostrictive with a dielectric constant thatchanges with applied electric field. Furthermore, barium strontiumtitanate is preferred.

In operation, the voltage at each array element will be the sum of thesignal on its face and back. The resulting strain will be the square ofthis sum. With the appropriate choice of front and back control signalfrequencies (row and column), only the sum (or difference) frequency andphase component will fall within the pass-band of the transducer and beradiated.

Electrostrictive transducers can also be used in receive providing thatthe transducer material has a large non-linear dielectric response aswell as a large electrostrictive response. The required row and columnmixing is performed by the non-linear dielectric response. In otherwords, an electric field is produced that is, for example, the square orsome other non-linear response of the electric field across thetransducer due to the non-linear dielectric response of the transducermaterial. In receive, the effect of electrostriction is to change thedielectric constant (permittivity) of the transducer as a function ofmechanical stress resulting from the incoming acoustic field. Parametricmixing takes place between the electric and acoustic fields to producean electro-acoustic signal. This produces a number of frequencycomponents (at least 4) only one of which will be the desired signal.Through a suitable choice of materials and control frequencies suchoperation can, create the desired 2-D focus of acoustic energy.

Referring now to the composite implementation, this implementationprovides several advantages, mostly stemming from its manufacture insemiconductor material that permits the incorporation of a large rangeof electronic circuits. Some of the advantages of the compositeimplementation are discussed herein. Also included is the possibility tointegrate the row and column control circuits 30,40 on the array chip.Such integration would reduce the number of array connections fromseveral hundred to a few dozen and significantly reduce the cost of thesystem. Integrating the array and the control circuits on a single chipalso permits manufacture of a portable imaging system low enough in costto be used in essentially all situations where volummetric and/or realtime 2-D imaging are required.

OPERATION

All phased array imaging systems image by electronically synthesizing alens. For 3-D imaging, one needs to synthesize a 2-D lens. In this case,the requisite delay over the aperture can be separated into twocomponents, one that depends only on X and one that depends only on Y astaught herein.

For apertures that are less than one half the focal length (this is thenormal operating condition for medical imaging), the lens equation canbe approximated by a function that is separable into independent X and Ycomponents. As seen in Equation 1, this is the classical paraxialapproximation; an approximation that is the foundation for FourierOptics as well as other field in wave mechanics.Δ(X,Y)≡(X²/(2*R)+Y²/(2*R)/V   Eq. 1

The paraxial approximation allows one to decompose a 2-D lens into two,orthogonal, 1-D lenses, one immediately in front of the other. At eachpoint on the aperture, the phase delay from one lens adds to that of theother to produce the same phase delay as would result from a single 2-Dlens. This means that a 2-D array can be used to synthesize a 2-D lensby phasing the rows with a phase relationship that will create a 1-Dfocus in the X-axis and the columns with one that will create a 1-Dfocus in the Y-axis. Such phasing is now discussed in more detail, firstin a general continuous wave context and then in a context for broadbandoperation using coded signals.

In general operation, the row signals are used to produce a 1-D focus inthe X direction while the columns signals are used to produce a 1-Dfocus along the Y axis. To achieve the desired 2-D focus, the row andcolumn signals are combined in a manner such as that achieved by mixing(or multiplying) the signals together. Such mixing allows one tosynthesize a 2-D focus using only as many control signals are there arerows and columns in array 100. In other words, this permits theeffective control of M×N transducers by M+N control signals. Thus, theexemplary 10,000 transducers in array 100 (with M=N=100) can becontrolled by 200 processing channels.

FIG. 2 shows several cells and transducers of the active 2-D array 100.Here each array element is connected to the output of its own electronicmixing circuit. One input of the each mixer is connected to an electrodethat is shared by all other array elements on a given row. Likewise, theother input is connected to the corresponding column electrode. Mixingthe external row and column signals together produces two signalcomponents at each array element, one that is the sum of the frequencyand phase of the row signal and the column signal, and the other whichis the difference. By choosing the frequency of the row and columnsignals such that only the difference (or sum) frequency is within thepass-band of the transducer ensures that only the difference (or sum)frequency (and phase) component will be radiated from the array.

In receive, the desired 2-D control signal is created in the first mixerfrom the external row and column control signals. This signal, in turn,is mixed with the received signal from the transducer. The resultantsignal is added to all others of the array elements of the compositestructure. If, as shown, the output of the first mixer is not filtered,the output of the second mixer will contain four frequency components,only one of which is the desired signal. If desired, filters may beadded in the composite implementation after each mixer. Though this willimprove performance, a trade-off exists as to whether the increasedperformance is sufficient to justify the added expense and complexity ofadding these devices. In the embodiment of FIG. 2, the desired componentis preferably filtered by band pass filter 310 after summation. In theembodiment for sector scanning and discrete dynamic focusing (FIG. 11,etc.), filters are preferably provided after each mixer to eliminateextra frequency components.

Together with array 100, control signal generators 30, 40 comprise thebeamforming process of system 10. The frequency and phase of the row andcolumn array control signals determine the focus and angle of thetransmit and receive beams in accordance with the equations herein.Having generally introduced transmit and receive operations, broadbandapplications is are now discussed.

Current imaging systems achieve sub-millimeter resolution by usingshort, high bandwidth pulses of acoustic energy. This same level of highbandwidth operation cannot be achieved in the M+N control line array 100of FIG. 2 using continuous wave signals. Accordingly, a coded signal orthe like, such as the linear FM chirp, is used to achieve high bandwidthand to thereby provide improved range resolution.

In transmit, the control signals are linear FM chirps having half thechirp rate of the transmitted acoustic signal (when they are mixed, thechirp rates add to produce a full chirp rate). The length of the chirpsare also longer, in time, than the acoustic signal. The length of eachcontrol signal chirp depends upon the transmitted chirp length and therequired delay for a particular focal point.

At each array element, the row signal and column signal are mixedtogether to produce several signals, one of which is a chirp that is thesum of the two control signals. By suitable choice of the base frequencyof the control signals, the desired chirp component will be within thebandwidth of the acoustic transducer while the other components willfall significantly outside of this band. It should be recognized thatthis chirp component lasts much longer in time than the desiredtransmitted signal as well as covering a frequency range longer than theresponse of the transducer. The bandwidth of the transducer and thechirp rate is chosen such that the acoustic chirp transmitted from theelement will be the desired length.

Referring to Equation 2, the relative timing of the transmit controlsignal at some array element is determined by the relative time oftransit from a specific array element (a particular row-column, X-Ylocation) and the desired focal point (X,Y,θ,Φ,R). X and Y are thespatial location of the element, θ and Φ are the azimuth and elevationdirection cosines of the ray connecting the center of the array to thefocal point, and R is the range from the center of the array to thefocal point. Using the paraxial approximation this simplifies toEquation 3.

Separating Equation 3 into X and Y components produces the row andcolumn control signals. Equations 4 and 5 respectively, for transmit. Inthese equations, ωr and ωc are the row and column base frequencies. α isthe chirp rate and V is the velocity of sound.Δ(X,Y)=sqrt(R²+X²+Y²−2XRθ−2YRΦ)/V−R/V   Eq. 2$\begin{matrix}\begin{matrix}{{\Delta\left( {X,Y} \right)} \cong {\left( {{{X^{2}/2}R} + {{Y^{2}/2}R} - {X\quad\theta} - {Y\quad\phi}} \right)/V}} \\{\cong {{\Delta(X)} + {\Delta(Y)}}}\end{matrix} & {{Eq}.\quad 3}\end{matrix}$Sr(X)=cos(ωr*(t−Δ(X))+α*r²/2−2α*Δ(X)*t+α*Δ(X)²)   Eq. 4Sc(Y)=cos(ωc*(t−Δ(Y))+α*r²/2−2α*Δ(Y)*t+α*Δ(Y)²)   Eq. 4

In receive, the purpose of the control signals which have thecharacteristics of continuous wave signals, is to shift the frequencyand phase of each signal so that, as that signal occurs, it coherentlyadds with all the other signals as they progress in their time sequence.The net result is, for a single point source, a single output chirpwhose length in time and frequency corresponds to the total time overwhich the acoustic energy is insonifing the array aperture. For a pointsource located at a large angle away from array 100, the resultingoutput chirp can last over 20 microseconds even though the chirp comingfrom the point source or target lasted only 10 microseconds.

Mathematically, this requirement to achieve coherence can be establishedby changing the phase and amplitude of every chirp so that the summedoutput produces a single chirp centered in time with the chirp signalarriving at the center element of the array. Equation 6 provides thecondition for coherence (ωa is the base frequency of the receivedchirp). Solving for the frequency shift ‘ωs*t’, and the phase shift ‘ψ’,gives the frequency and phase shift for each array element to ensure acoherent sum, Equation 7. Separating this equation into its row andcolumn components gives rise to the row and column control signals,Equations 8 and 9 respectively, for receive (ωlor and ωloc are the rowsand column local oscillator frequencies and tz is the transit time fromthe center of the array 100 to the target).cos(ωa*(t−tz)+α*(t−tz)²)=cos(ωΔ*(t−Δ(X,Y))+α*(t−Δ(X,Y))²+ωr*t+ψ)   Eq. 6ωs=2*α*Δ(X,Y)ψ=ωa*Δ(X,Y)−α*(Δ(X,Y))²   Eq. 7Sr=cos(ωlor*t+2*α*Δ(X)*t+ωa*Δ(X)−α*(Δ(X)²))   Eq. 8Sc=cos(ωloc*t+2*α*Δ(Y)*t+ωa*Δ(Y)−α*(Δ(Y)²))   Eq. 9Dynamic Focusing

In current pulsed array imaging systems, a single pulse of acousticenergy is transmitted from an array for every line of range data that iscollected. As that pulse travels away from the aperture it interactswith progressively deeper objects (targets). For best resolution, thefocal length of the system is dynamically changed to follow the pulse asit interacts with objects at ever increasing ranges. This process isknown as dynamic focusing and is one of the main advantages of arraytechnology. Furthermore, dynamic focusing and discrete dynamic focusing,discussed below for sector scanning, permit the generation of real timeimages.

For linear or curved linear scanning or planar or curved planar scanningas taught herein, the direction cosines are zero. Without correction,the response of the system will fall away from the chosen focal range.To correct for this problem, the control signals must change in time insuch a way as to keep all points in range in focus as they aresequentially insonified by the transmit chirp. Since the acoustic energymust travel from the array to some target and back again, the effectiverate at which the targets are insonified is ½ the speed of sound. Thusto keep targets at different ranges in focus, the system must increaseits focal distance at ½ the speed of sound.

Due to the time dependence of the focal changes, simply substitutingR=½*V*t into the control signals gives rise to a DC signal and is notadequate. Appropriate delay, δ(t), is determined by the frequency of thecontrol signals, Equation 10. The phase evolution of the control signalscan be found by integrating this frequency shift, ωs(t), as demonstratedin Equations 11 and 12. To determine the constants of integration, sometime ‘Tm’ is chosen to be the beginning point of the dynamic focusingprocess. The constant of integration is found by setting equations equalto Equations 11 and 12 at the time Tm. The result, for dynamic focusingis the control signals described by Equations 13 and 14.δ(t)=ωs/(2*a)ωs(t)=2*a*(X²/(V*t)+Y²/(V*t))   Eq. 10$\begin{matrix}\begin{matrix}{{{Phase}(x)} = {{integral}\left( {2*\alpha*{\Delta\left( {X,t} \right)}{dt}} \right)}} \\{= {{2*\alpha*X^{2}*{{in}(t)}} + {K1}}}\end{matrix} & {{Eq}.\quad 11}\end{matrix}$ $\begin{matrix}\begin{matrix}{{{Phase}(y)} = {{integral}\left( {2*\alpha*{\Delta\left( {Y,t} \right)}{dt}} \right)}} \\{= {{2*\alpha*Y^{2}*{{in}(t)}} + {K2}}}\end{matrix} & {{Eq}.\quad 12}\end{matrix}$Sr=cos(ωlor*t−2*α*X²*In(t/Tm)+2*α*Δ(X,Tm)+ωa*Δ(X,Tm)−α*Δ(X, Tm)²)   Eq.13Sc=cos(ωlor*t−2*α*Y²*In(t/Tm)+2*α*Δ(Y,Tm)+ωa*Δ(Y,Tm)−α*Δ(X, Tm)²)   Eq.14

It should be noted that at scan angles more than a few degree,continuous dynamic focusing is not possible. This is due to the 2XRθ and2YRΦ terms in Equation 2. The rate of change in the focal length causesthese terms to introduce a frequency shift that significantly degradesthe output signal.

Planar Array

Referring to FIG. 5, a diagram of a planar array 505 in accordance withthe present invention is shown.

Row control lines 35 and column control lines 45 are connected to theplanar array 505 to deliver the appropriate control signals discussedherein and the collective output signal from transducers (not shown) inarray 505 is propagated on line 65 to the interface circuit 20.

Planar scanning is achieved by setting the direction cosines to zero inEq. 2 and electrically translating a sub-aperture 510 across the array505. Electrically translating a sub-aperture is generally known and itsimplementation in system 10 would be apparent to one skilled in the artgiven the teachings herein. Similar scanning in either an X or Ydirection in 1-D arrays has been termed “linear” scanning. The term“planar” scanning is used herein to denote scanning a sub-aperture inboth the X and Y directions in a 2-D array. Though the array 505 andsub-aperture 510 may have any practical dimension, in one practicalembodiment the X and Y dimensions of the array 505 are approximatelyeach 4″ and the dimensions of the X and Y sub-aperture 510 areapproximately each ¾″.

Curved Planar Array

Referring to FIG. 6a, a convex curved planar array 550 and a less 555therefor in accordance with the present invention are shown. The controllines and processing circuitry (not shown) for the array 550 are astaught herein.

Providing that the curved planar array 550 is not excessively curvedrelative to its active sub-aperture, for example for a sub-aperture of1.5 cm a curved planar array with a curvature of r=4 cm is suitable,dynamic focusing can be achieved at a large angle by electronicallytranslating a sub-aperture over array 550. Electrically translating asub-aperture across a curved array is generally known. By the curvatureof the array 550, the sub-aperture is able to scan an angle with thedirection cosines equal to zero.

The less 555 provides focal point adjustment. For example, without theacoustic lens 550, an electronic focal length of 3 cm would correspondto an acoustic length of 6 cm due to a 4 cm convex curvature of array550. Using a lens 550 having an acoustic velocity 0.8 that of water, theacoustic focal length is reduce to 4 cm. The convex shape of array 550acts as a diverging lens. The acoustically slow convex covering 550 actsas a diverging lens that removes some of the diverging curvature of thewavefront. The array curvature has a significantly less pronouncedaffect at a 9 cm focal length. It should be recognized that although adiverging lens 555 is shown, a converging lens or no lens at all may beutilized.

Referring to FIG. 6b, a concave curved planar array 570 in accordancewith the present invention is shown. An acoustic lens 575 is alsoprovided for focusing acoustic energy from array 570.

Curved planar scanning can be achieved in both the composite andnon-linear implementations.

Sector Scanning

As noted above, continuous dynamic focusing is achievable when angularor sector scanning is not performed. Discontinuous or discrete focusinghowever, can be achieved in angular scanning systems at a level thatapproximates continuous focusing if additional electronic componentry(discussed below) is added to system 10. The additional electroniccomponentry is implemented in the semiconductor material of thecomposite implementation, but the functions it provides are notproperties of non-linear electro-acoustic material. Accordingly, sectorscanning can be achieved only in the composite implementation.

Sector scanning requires that the transmit and receive beams be scannedover a predefined angle, normally +/−45 degrees (direction cosines+/−0.5 and +/−0.5, azimuth and elevation, i.e., X and Y). Increasing thepointing angle to 45 degrees in both azimuth and elevation significantlydegrades the response. To correct for this distortion, an additionalcross term is required and it is:Err=(X*Y(θ*Φ)/(R*V)   Eq. 16As this term contains information unique to the X and Y position of agiven, it cannot be incorporated into row and column control signals.This is why the non-linear array is not effective at large angles.

Referring to FIG. 7, a schematic diagram of two transducer cells withphase adjustment for angular scanning in accordance with presentinvention is shown. The two cells 610 and 640 are analogous to cells 110and 140, for example, of FIG. 2.

Referring to cell 610, the first mixer 611, buffer 612, T/R switch 613,amplifier 617 and second mixer 618 are analogous to their counterpartsin cell 110. Cells 610, 640 each include a phase shifter 614, 644 and avoltage divider 616, 646. A DC signal source 605 for generating a commonDC control signal is connected to the voltage divider. It is controlledby an additional processing channel (not shown).

The limitations imposed by Eq. 15 are removed by the addition of thephase shifters 614, 644 as programmed by the voltage divider outputs.The voltage dividers 616, 646 essentially comprise two resistors thatcan be precisely selected to divide the common DC signal to a uniquevoltage level. This voltage level or ratio of input to output voltage ischosen for each cell to be proportional to its XY position in the array(100 of FIG. 2). Eq. 16 shows the relationship of the DC control signaland Eq. 17 shows the actual phase shift introduced by each phase shifterin array 100 (FIG. 2), represented in FIG. 7 by phase shifters 614 and644.Scorr=(θ+Φ)*(ωa+2*a*R/V)/(2*R*V)   Eq. 16C=X*Y*Scorr   Eq. 17

The immediately preceding discussion illustrated a way of achievingunique phase correction for each transducer for achieving angular scanin transmit. A way of angularly focusing in receive is now discussed.

Prior art acoustic imaging systems sample the output image at discreteranges. For this reason, a continuous output, in range, is not required.One can use a sequence of range outputs, in other words, discontinuousor discrete dynamic focusing, without any loss in image quality that isdetectable by the human eye.

In the composite implementation, discrete focusing is achieved by atleast the two following approaches or a combination thereof. A firstapproach is to use as many processing electronic cells per transducer asthe number of range increments desired. The control signals only have tobe in existence for the duration over which the energy from a particularrange point insonifies the array. This concept is illustrated in FIG. 8,wherein dashed line 681 represents a ray or line emanating from thecenter of array 100 on which range points for focusing lie. The ray 681is defined by certain elevation and azimuth angles. Segment 683represents one process period which is essentially the time over whichenergy from a focused range point insonifies the array. The range focusalong ray 681 is sequential extended a distance equal to the speed ofsound in the relevant medium times the period of insonification, up to adistance that is no longer practical or desirable for scanning. For apractical design for use in medical ultrasound imaging, the processperiod is on the order of 20 microseconds. This means that every 20microseconds, the control signals can change to focus on a new rangepoint. Having 40 processing cells for every array element would permitone range sample every 0.5 microseconds; approximately what is used forcurrent imaging systems when displaying 16 cm of range.

A second approach to obtain multiple range samples is to use the highelectronic bandwiths of current integrated electronic circuits. Assuminga bandwidth requirement of 10 MHz per range channel, a 400 MHzelectronic bandwidth would permit 40 simultaneous range channels.Implementation of this approach in the imaging system 10 describedherein is now presented.

Referring to FIG. 9, a range versus frequency band diagram forimplementing discrete focusing is shown. A plurality of range points aredefined, point 1, point 2, . . . point j, that sufficiently approximatethe range overwhich focusing is desired along a particular ray (681 ofFIG. 8). A specific frequency band, band 1, band 2, . . . band j, isdefined for each range point.

Referring to FIG. 10, a phase accumulator 691 is provided either in orin communication with the interface circuit 20. The phase accumulator691 preferably receives a digital signal, ν(t), from signal generatingcircuitry (not shown, but generally known), in system control circuitrythat includes components for each of the j frequency bands of FIG. 9.Thus,ν(t)=ν(t)₂′+ν(t)₂′+. . . +ν(t)_(j)′  Eq. 19where ωlor₁+ωloc₁=band ν(t)₁ center frequency and ωlor₂+ωloc₂=band ν(t)₂center frequency, etc.

The phase accumulator 691 preferably includes a digital to analogconverter (not shown) or one is placed downstream thereof. The output ofaccumulator 691 is the Scorr signal which is delivered to the voltagedividers (616,646 of FIG. 7). The output of each voltage divider is thecontrol signal, C, which is propagated to the phase shifters (614, 644of FIG. 7) to uniquely code the receive focusing signal for each cell.In the exemplary embodiment, mentioned immediately above, each band orfrequency component ν(t)′ differs by 10 MHz from the adjacent band.Thus, for 40 range channels, ν(t) has a band width of 40×10 MHz=400 MHz.

Referring to FIG. 11, a modification in the interface circuit 20 toappropriately process a multi frequency component signal in accordancewith the present invention is shown.

In contrast to the singular receive channel 305 of the embodiment ofFIG. 3, the embodiment of FIG. 11 includes j receive channels 705 (705₁, 705 ₂, 705 _(j)) which contain matched filters 720 ₁, 720 ₂, 720 _(j)that are specifically configured for their corresponding frequencycomponent ν(t)₁′, ν(t)₂′, ν(t)_(j)′, respectively. Continuing with thecurrent example of 40 range points and 40 frequency components, thereare 40 receive channels 705 in the modification to the interface circuit20 illustrated in FIG. 11. It should be recognized that one can combinemultiple cells per transducer, for example 6 cells per transducer (withappropriate frequency multiplexing and phase shifting as taught herein),with larger bandwidth signals, for example 6 frequencies in thebandwidth and 6 receive channels, to achieve the desired number of rangesamples, in this example, 6×6=36.

1-D Implementation and Annular Array

Referring to FIG. 12, a 1-D array 800 for an acoustic scanning system inaccordance with the present invention is shown. The array 800 isintegrated into the system 10 of FIG. 1, replacing array 100. Sincearray 800 is one dimensional, control lines are only implemented in onedimension, either row or column control. The row control circuiting 30is shown in FIG. 12 and hence in integrating array 800 into system 10,the column circuit 40 and related electronics are removed. Each cell810,840,870 does not contain a first mixer, such as mixer 111 and thelike of FIG. 2 because of the absence of column control lines, but doesinclude a buffer 812,842,872, a T/R switch 813,843, 873 receiveamplifier 817,847,877 and a second mixer 818, 848,878. Each cell isconnected to a transducer 815,845, 875. Cells 810,840,870 are otherwisegenerally analogous to cells 110,140,170 of FIG. 2. Accordingly, theymay be implemented as a composite array or non-linear array and beconfigured in embodiments for linear and curved linear scanning in bothimplementations, and for sector scanning in the compositeimplementation. In addition, the 1-D array can be implemented as adiscrete array coupled to discrete electronics. The 1-D array 800operates under the same signal generating and processing aspects taughtherein, with the exception of those aspects specific to the mixing ofrow and column signals.

It should also be recognized herein that the teachings herein apply toannular arrays and they could, accordingly, be fabricated in the sameembodiments of a 1-D array discussed immediately above.

Aberration Correction

The various tissues of the body have differing speeds of sound. Notcorrecting the beamforming process for this fact may degrade the acuityof the resulting acoustic images. To date, modest improvement in imagequality has been achieved by aberration correction techniques on 1-Dphased arrays. It is generally accepted, however, that these methodswould produce a significant increase in image quality if they could beapplied to large 2-D apertures. Thus, perhaps the most significantimpediment to aberration correction has been the expense and complexityof building a 2-D array imaging system.

Use of the present invention makes practical a 2-D scanning system andhence makes possible aberration correction techniques. The requireddelay perturbations are achieved by suitable modification of the row andcolumn control signals. In a relatively basal embodiment, the system canimplement any delay profile that is separable into X and Y components.Since the present system has been shown to correct the large first andsecond order delays required to produce an image in homogeneous media,the constraints of separability upon the improvement in image qualityand general system performance should not be significant.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention and the limits of the appended claims.

1. An acoustic imaging apparatus, comprising: control logic; a pluralityof transducer elements arranged in an array, each coupled to saidcontrol logic and capable of transmitting an acoustic signalrepresentative of an electrical transmit control signal propagated fromsaid control logic and generating an electrical receive signalrepresentative of an incident acoustic signal; means within said controllogic for generating an electrical transmit control signal for eachtransducer element such that the electrical transmit control signal foreach transducer element contains a coded signal; means within saidcontrol logic for generating an electrical receive control signal foreach transducer element such that the electrical receive control signalfor each transducer element contains a frequency and phase shift thanwhen combined with the transducer element's electrical receive signalmodifies the frequency and phase of that electrical receive signal insuch a manner as to permit the coherent combination of the modifiedelectrical receive signals from all of said plurality of transducerelements; means for combining the electrical receive control signal ofeach transducer element with an electrical receive signal generated bythat transducer; means coupled to each of said transducer elements forcombining the modified electrical receive signals from said transducerelements so as to form a coherently combined array output signal; meanscoupled to said transducer output combining means for decoding acombined reflected coded signal in the coherently combined array outputsignal to produce a decoding means output signal; and means coupled tosaid decoding means for generating image data from said decoding meansoutput signal.
 2. The apparatus of claim 1, wherein said coded signal isa chirp.
 3. The apparatus of claim 2, wherein said decoding meanscomprises at least one matched filter for coded signal decoding.
 4. Theapparatus of claim 1, wherein said chirp is a linear FM chirp.
 5. Theapparatus of claim 1, wherein said array has a size of M rows and Ncolumns and said electrical transmit control signal generating meanscomprises means for generating individual row and column transmitcontrol signals for each of said rows and columns, the electricaltransmit control signal for each transducer element being a combinationof the transmit row and column control signals for that transducer. 6.The apparatus of claim 5, wherein at least one of said row and columntransmit control signals for a given transducer element contains afrequency based coded signal.
 7. The apparatus of claim 5, wherein saidelectrical receive control signal generating means comprises means forgenerating individual row and column receive control signals for each ofsaid rows and columns, the electrical receive control signal for eachtransducer being a combination of the receive row and column controlsignals for that transducer.
 8. The apparatus of claim 1, wherein saidcoded signal includes a frequency based code.
 9. The apparatus of claim1, wherein said array is a one dimensional array with a plurality ofrows and one column.
 10. The apparatus of claim 1, wherein said array oftransducer elements comprises M rows and N columns, where M and N arepositive integers and at least one of M and N is greater than 1; atleast one of said transmit control signal generating means and saidreceive control signal generating means includes means for generatingrow and column control signal components; and wherein each transducerelement includes an active electronic device for combining said row andcolumn control signal components for that transducer element.
 11. Theapparatus of claim 1, wherein each transducer element includes atransducer comprised of a non-linear electro-acoustic, non-lineardielectric material.
 12. An acoustic imaging apparatus, comprising: aplurality of electro-acoustic transducer elements arranged in an array,each capable of transmitting an acoustic signal and generating anelectrical signal representative of an incident acoustic wave; controlmeans having a plurality of control channels coupled to each of saidplurality of transducer elements, said control channels being fewer innumber than said transducer elements; wherein said control meansgenerates control signals for each transducer element that when combinedwith the electrical receive signal of that transducer element modifiesthe electrical receive signal in such a manner as to permit thesimultaneous processing of the modified electrical receive signals fromsaid plurality of transducer elements; means for combining the modifiedelectrical receive signals of each of said transducer elements to forman array output signal; and means coupled to said combining means forgenerating image data from said array output signal.
 13. The apparatusof claim 12, wherein said array has a plurality of rows and a pluralityof columns each having one of said plurality of control channelsassociated therewith; said control signal generating means furtherincluding means for generating row and column control signal components;and wherein said transducer element is uniquely and simultaneouslycontrolled by a combination of the row and column control signalcomponents for that transducer element.
 14. The apparatus of claim 12,wherein said control signal generating means further includes means forgenerating a transmit control signal for each transducer element thatcontains a frequency based coded signal for transmission by eachtransducer element.
 15. The apparatus of claim 14, further comprisingmeans for decoding a reflected frequency based coded signal.
 16. Anacoustic imaging system, comprising: an array of electro-acoustictransducer elements having M rows and N columns, where M and N arepositive integers and at least one of M and N is greater than one; M rowcontrol lines, each coupled to the transducer elements in one of said Mrows; N column control lines, each coupled to the transducer elements inone of said N columns; control means coupled to each of said M row and Ncolumn control lines for generating row control signals for each of saidrow control lines and column control signals for each of said columncontrol lines, a control signal for each transducer being a combinationof one of said row control signals and one of said column controlsignals; a plurality of active devices, each coupled to one of saidtransducer elements for combining the row control signal and the columncontrol signal of that transducer element; means for combining theoutput of each transducer element to produce an array output signal; andmeans coupled to said transducer output combining means for generatingimage data from said array output signal.
 17. The apparatus of claim 16,wherein said active device is an active electronic device.
 18. Theapparatus of claim 17, wherein said control means includes means forgenerating a transmit control signal that contains a frequency basedcoded signal for each transducer element; and wherein said apparatusfurther comprises means in communication with each of said transducerelements for modifying a reflected coded signal received thereby toachieve a delay encoded in the coded signal, said delay for eachtransducer element being based on the relative position of thattransducer element in the array.
 19. The apparatus of claim 16, whereinsaid active device includes a non-linear electro-acoustic material. 20.The apparatus of claim 16, wherein said active device includes anon-linear electro-acoustic material for combining row and columncontrol signal on transmit and an active electronic device for combiningrow and column control signal on receive.
 21. The apparatus of claim 16,wherein said active device includes a non-linear electro-acoustic,nonlinear dielectric material.
 22. A method for acoustic imaging,comprising the steps of: providing control logic; providing a pluralityof transducer elements arranged in an array, each coupled to saidcontrol logic and capable of transmitting an acoustic signalrepresentative of an electrical transmit control signal propagated fromsaid control logic and generating an electrical receive signalrepresentative of an incident acoustic signal; generating an electricaltransmit control signal for each transducer element such that theelectrical transmit control signal for each transducer element containsa coded signal; generating an electrical receive control signal for eachtransducer element that contains an appropriate frequency and phaseshift that when combined with that transducer element's electricalreceive signal permits the coherent combination of the electricalreceive signals of each of the plurality of transducer elements;combining the coherent output signals from said transducer elements soas to form a coherently combined array output signal; decoding acombined reflected coded signal in the coherently combined array outputsignal to produce a decoded output signal; and generating image datafrom the decoded output signal.
 23. An acoustic imaging apparatus,comprising: control logic; a plurality of transducer elements arrangedin an array, each coupled to said control logic and capable oftransmitting an acoustic signal representative of an electrical transmitcontrol signal propagated from said control logic and generating anelectrical receive signal representative of an incident acoustic signal;means within said control logic for generating an electrical transmitcontrol signal for each transducer element that contains a frequencybased coded signal and cause causing each transducer to emit an acousticsignal representative of said coded signal; means for modifying thefrequency and chase phase of an electrical receive signal of eachtransducer element for coherently combining reflected coded signalswithin the electrical receive signals thereof; means coupled to saidmodifying means for decoding the combined reflected coded signals toachieve a time delay based on that coded signal; and means coupled tosaid decoding means for generating image data from an output signaltherefrom.
 24. An acoustic energy transmitting apparatus, comprising: aplurality of electro-acoustic transducer elements arranged in an M rowby N column array, where M and N are positive integers and at least oneof M and N is greater than one; control circuit for propagating row andcolumn control signals for each of said M rows and said N columns, eachcontrol signal having a frequency and a phase component; and whereineach transducer element is configured to function as an active device soas to achieve a combining at each transducer element of the frequencyand phase components of the row and column control signals for thattransducer element in such a manner as to provide a focused acousticsignal at a given focal distance and direction from said array.
 25. Theapparatus of claim 24, wherein the electric signal to acoustic signalrelationship and vice versa of each transducer element is non-linear.26. The apparatus of claim 24, wherein said control circuit includes acontrol channel for each of said M rows and a control channel for eachof said N columns, and wherein the number of control channels is fewerthan the number of transducer elements.
 27. The apparatus of claim 24,wherein said control circuit is configured such that the row and columnsignals for at least some of the transducer elements includes a codedsignal.
 28. The apparatus of claim 27, wherein M equals one.
 29. Anacoustic energy transmitting apparatus, comprising: a plurality ofelectro-acoustic transducer elements arranged in an M row by N columnarray, where M and N are positive integers and at least one of M and Nis greater than one; M row control lines, each coupled to the transducerelements in one of said M rows; N column control lines, each coupled tothe transducer elements in one of said N columns; control circuit forpropagating row and column control signals for each of said M rows andsaid N columns, a control signal for each transducer element being acombination of one of said row control signals and one of said columncontrol signals; a plurality of active devices, each coupled to one ofsaid transducer elements for combining the row control signal and thecolumn control signal of that transducer element; wherein saidtransducer elements, control circuit and active devices are configuredso as to achieve a combining at each transducer element of the row andcolumn control signals for that transducer element in such a manner asto provide a focused acoustic signal at a given focal distance anddirection from said array; and wherein each of said electro-acoustictransducer elements is configured within said apparatus to function in anon-linear manner in operation.
 30. An acoustic energy receivingapparatus, comprising: a plurality of electro-acoustic transducerelements arranged in an M row by N column array; control circuit forpropagating row and column control signals for each of said M rows andsaid N columns, each row and column control signal having a frequencyand a phase component; and wherein said transducer elements and saidcontrol circuit are configured so as to achieve a combining at eachtransducer element of the frequency and phase components of the row andcolumn control signals for that transducer element with a resultantelectrical receive signal, corresponding to an acoustic signal incidenton that transducer element, in such a manner as to modify the frequencyand phase of the transducer element's electrical receive signal so as toachieve the coherent combination of the modified electrical receivesignals from all of said plurality of transducer elements; and a filterthat filters spurious frequencies output from the transducer elements;wherein said transducer elements, control circuit and filter areconfigured to achieve focused acoustic signal reception at a givendistance and direction from said array.
 31. The apparatus of claim 30,wherein said transducer elements and said control circuit are configuredto achieve dynamic focused acoustic signal reception.
 32. The apparatusof claim 31, wherein the electric signal to acoustic signal relationshipand vice versa of each transducer element is non-linear.
 33. Theapparatus of claim 30, wherein said filter includes a matched filter.34. The apparatus of claim 33, wherein said matched filter includes aconjugate of a coded signal.
 35. The apparatus of claim 29, wherein Mequals one.
 36. The apparatus of claim 30, further comprising a circuitthat generates image data from the coherent combination of transducerelement receive signals.
 37. The apparatus of claim 30, wherein saidcontrol circuit includes a control channel for each of said M rows and acontrol channel for each of said N columns, and wherein the number ofcontrol channels is fewer than the number of transducer elements.
 38. Anacoustic energy receiving apparatus, comprising: a plurality ofelectro-acoustic transducer elements each capable of generating anelectrical receive signal in response to an incident acoustic wave andarranged in an M row by N column array, where M and N are positiveintegers and at least one of M and N is greater than one; controlcircuit for propagating row and column control signals for each of saidM rows and said N columns, the control signal for each transducerelement being a combination of the row and column control signals forthat transducer element; wherein said row and column control signals areconfigured, for each transducer element, such that when combined withthe electrical receive signal of that transducer element the electricalreceive signal is modified in such a manner as to permit thesimultaneous processing of the modified electrical receive signals fromsaid plurality of transducer elements; a first circuit that combines themodified electrical receive signals of each of said transducer elementsto form an array output signal; and a second circuit coupled to saidfirst circuit that generates image data from said array output signal.39. The apparatus of claim 38, wherein M equals one.
 40. The apparatusof claim 24, wherein each transducer element includes non-linearelectro-acoustic material.